Eater of knowledge
Horrid, disturbing creatures, the eaters of knowledge are the agents of Ilsensine in the greater Outlands and the planes beyond, handling those tasks for which the god-brain does not require the stealth of cranium rats or the will of the illithid; entering places where Ilsensine cannot otherwise extend its awareness, and collecting knowledge either through observation, or by harvesting and incorporating the brains of others into its form. At times, they are even sent against the illithid themselves, serving as living weapons against any that may have displeased Ilsensine.
Eaters of knowledge are utterly without will, existing for nothing but to be the extension of Ilsensine's desire for knowledge. While they do have minds of their own, they have literally no ability to do anything but what their master orders them to, merely granted intelligence that they may enact its will in the most appropriate means possible for any given situation.
Somewhere between constructs and undead, eaters of knowledge have no need for food or drink, no life cycle, no natural means of reproduction. While they do feed on the brains of their victims, they draw no true sustenance from the process, nourished only on the memories and experiences locked within those minds. Only sentient memories and experiences at that; they can gain nothing at all from consuming the brain of an animal.
These beings are spawned only by Ilsensine itself, created from the brainless husks left by those that have succumbed to its presence within the Caverns of Thought. The details have never been observed by any willing to speak, but most greybeards believe it involves the husk's dead brain being replaced with a small portion of Ilsensine's own. it's unknown how much these creatures truly require any degree of nourishment, as they seem to be immune to any sort of natural death; an eater of knowledge exists until it no longer needs to, at which point it is destroyed.
Horrifying in appearance, an eater of knowledge looks like nothing more than a nine-foot tall, vaguely humanoid-shaped mass of brain, sinew, tendon, and hide, dripping a foul black ooze and constantly surrounded by the sickly-sweet scent of rotting tissue. The surface of its exposed grey matter twitches , coated in extremely fine nerve endings that it can use to strike against the minds of its foes, growing into a victim's nervous system and extracting their brains bit by bit. These beings don't even have enough of a form to speak; though they seem to understand nearly any language they encounter, they never respond with anything more than a disturbing gurgle of moist flesh.
- Planescape Monstrous Compendium Appendix II, pp.26-27